1. Field of the Invention
The present invention relates to a process for producing a granular (or powdered) detergent composition containing a relatively large amount of a zeolite. More specifically, it relates to a process for producing a granular detergent composition having a small bulk density despite a relatively large content of a zeolite.
2. Description of the Prior Art
Phosphates have been widely used as builder components for granular (or powdered) detergent compositions. However, the use of phosphates in detergent compositions has been recently restricted from the viewpoint of pollution. For this reason, various attempts have been made in the art to develop a new detergent builder component to take the place of phosphates. For instance, alkali builders such as silicates and carbonates and organic builders such as citrates have been proposed as new builders, and detergent compositions containing the same have been developed. However, these compositions have disadvantages in that the detergency or detergent power thereof is not sufficient in water having a high hardness, although these compositions exhibit an acceptable detergency in water having a low hardness.
It is known in the art, as disclosed in British Pat. Nos. 1473201, 1473202, 1429143 and 1498492 that a zeolite (i.e., an aluminosilicate) is capable of capturing a Ca ion in hard water and, therefore, renders a detergent composition effective even in hard water. The zeolite is a builder having an excellent capability to soften hard water. However, granular detergent compositions containing zeolite, especially containing 10% by weight or more of zeolite, have a large bulk density.
It is known that the bulk density of a household granular detergent composition is one of the most important factors affecting the purchasing preference of consumers. This is because consumers tend to judge the amount of a detergent composition by volume rather than weight and because a detergent composition having a large bulk density is deemed to be a small amount as compared with one having a small bulk density, in spite of being the same weight.
Various attempts have been made to decrease the bulk density of granular detergent compositions containing a zeolite. Some known methods are to increase the water content of the detergent composition slurry to be spray dried or to raise the temperature of hot air during the spray drying. However, the former method results in an undesirable drying load due to the increase in water, and the latter method is liable to lower the quality of the detergent composition due to the high temperature exposure.
Furthermore, it is known in the production of granular detergent compositions containing a relatively large amount of a phosphate but no zeolite that a gas such as air can be previously introduced or bubbled into the detergent slurry to be spray dried so as to control the bulk density of the granular detergent composition thus obtained (see U.S. Pat. Nos. 3,629,951 and 3,629,955, Japanese Patent Application Laid-Open (Kokai) No. 52-133166. However, it has been confirmed that simple application of this method to the production of a detergent composition containing a zeolite and a small amount of a phosphate results in undesirable free flowability and compressive hardening resistance unless the amount of the detergent slurry sprayed per hour is remarkably decreased, although the bulk density per se of the resultant granular detergent composition can be controlled.